MyShell - Custom Unix Shell Implementation

A basic Unix shell implementation written in C that supports core shell functionality including command execution, built-in commands, and I/O redirection.

Features Implemented

Core Shell Functionality

• Interactive Command Prompt: Displays current working directory in the format @LaxmanGupta(Myshell):<current_directory> >>
• Command Line Parsing: Handles whitespace, arguments, and quoted strings
• Process Creation: Uses fork() and execvp() for command execution
• Process Management: Proper parent-child process handling with wait()

Built-in Commands

• cd <directory>: Change current working directory
  • Handles directory change errors with appropriate error messages
• exit: Gracefully exits the shell with a goodbye message
• jobs: List all active background processes

Input/Output Redirection

• Input Redirection (<): Redirect stdin from a file
  • Example: cat < input.txt
• Output Redirection (>): Redirect stdout to a file (overwrites)
  • Example: ls > output.txt
• Proper file descriptor management and error handling

Pipe Support

• Command Pipelines (|): Chain multiple commands together
  • Example: ls -la | grep .txt | wc -l
• Multi-stage Pipelines: Support for multiple pipes in a single command
  • Example: cat file.txt | grep pattern | sort | uniq
• Proper inter-process communication using Unix pipes

Background Process Support

• Background Execution (&): Run commands in the background
  • Example: sleep 10 &
• Job Control: Track and manage background processes
• Job Listing: Use jobs command to see active background processes
• Automatic notification when background jobs complete

Signal Handling

• Ctrl+C (SIGINT): Interrupt foreground processes without killing the shell
• Ctrl+Z (SIGTSTP): Suspend foreground processes
• Child Process Management: Automatic cleanup of zombie processes
• Background processes ignore interrupt signals

Command Parsing Features

• Quote Handling: Supports quoted arguments with spaces
• File Redirection Parsing: Automatically detects and handles < and >
• Argument Tokenization: Splits commands into proper argument arrays

Error Handling

• Fork failure detection
• File opening errors for redirection
• Directory change errors
• Command execution failures
• Input validation and edge case handling

Technical Implementation

System Calls Used

• fork() - Process creation
• execvp() - Program execution
• wait() / waitpid() - Process synchronization
• chdir() - Directory changes
• open(), close() - File operations
• getcwd() - Current directory retrieval
• pipe() - Inter-process communication
• dup2() - File descriptor duplication
• signal() - Signal handling
• kill() - Send signals to processes

Memory Management

• Dynamic memory allocation with strdup()
• Proper string handling and null termination
• Buffer overflow protection

Build Instructions

Compile the Shell

make shell

Compile Optimized Version

make shell_optimized

Run the Shell

make run

Usage Examples

Basic Commands

@LaxmanGupta(Myshell):/home/user >> ls
@LaxmanGupta(Myshell):/home/user >> pwd
@LaxmanGupta(Myshell):/home/user >> date

Built-in Commands

@LaxmanGupta(Myshell):/home/user >> cd Documents
@LaxmanGupta(Myshell):/home/user/Documents >> jobs
@LaxmanGupta(Myshell):/home/user/Documents >> exit

I/O Redirection

@LaxmanGupta(Myshell):/home/user >> ls > filelist.txt
@LaxmanGupta(Myshell):/home/user >> cat < input.txt
@LaxmanGupta(Myshell):/home/user >> grep "pattern" < data.txt > results.txt

Pipes

@LaxmanGupta(Myshell):/home/user >> ls -la | grep .txt
@LaxmanGupta(Myshell):/home/user >> cat file.txt | grep pattern | sort | uniq
@LaxmanGupta(Myshell):/home/user >> ps aux | grep shell

Background Processes

@LaxmanGupta(Myshell):/home/user >> sleep 10 &
[1] 12345
@LaxmanGupta(Myshell):/home/user >> jobs
Active background jobs:
[1] 12345    sleep 10 &
@LaxmanGupta(Myshell):/home/user >> 
[1] Done    sleep 10 &

Quoted Arguments

@LaxmanGupta(Myshell):/home/user >> echo "Hello World"
@LaxmanGupta(Myshell):/home/user >> mkdir "My Documents"

Project Structure

shell/
├── src/
│   └── shell.c          # Main shell implementation
├── bin/                 # Compiled binaries (created after build)
├── notes/              # Documentation and implementation notes
│   ├── Pipes.md
│   ├── Fork.md
│   ├── Exec.md
│   ├── Wait_Dup.md
│   ├── Signal.md
│   └── Makefile.md
├── Makefile            # Build configuration
└── README.md           # This file

Current Limitations

Not Yet Implemented

• Environment variable expansion ($VAR)
• Command history
• Tab completion
• Append redirection (>>)
• Advanced job control (fg, bg commands)

Notes and Documentation

This repository includes detailed implementation notes:

• About Pipes
• About Fork
• About Exec
• About Wait and Dup
• Signals in C
• Makefile Guide

Author

Laxman Gupta

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This shell demonstrates fundamental Unix system programming concepts including process management, file I/O, and system call usage in C.